There are various situations which involve measuring the parameters of a rotary member, which can be referred to as rotational truth and plane truth. Rotational truth means concentricity of a peripheral surface of the rotary member, with deviations from a condition of such concentricity being referred to herein as radial run-out. Plane truth means that a lateral surface of the rotary member runs in the same plane so that that surface does not perform a wobbling movement in a lateral direction or in the axial direction of the rotary member, and deviations from a condition of truth in that respect will be referred to herein as lateral run-out. A typical situation which involves measuring radial and/or lateral run-out of a rotary member is that of testing a motor vehicle wheel, with or without a tire thereon, and such a measuring method is to be found in the German publication Werkstatt und Betrieb 103 (1970), pages 183-188. In that method, the rotary member testing operation involves mounting the rotary member in the testing machine by means of a play-compensating clamping arrangement and rotating the rotary member. During the rotary movement the rotary member is sensed with contact-type or non-contact sensors. When the rotary member is a motor vehicle wheel, the sensors can scan the wheel flanges of the disk wheels and tire tread strip portions and side walls of the pneumatic tire on a wheel. The results of the sensing operation or operations are then used to produce by mechanical or electrical means diagrams which show the radial and/or lateral run-out of the rotary member at the sensed locations thereon.
With measurement systems of that kind, it is not possible to avoid the clamping center of the clamping mounting arrangement being displaced relative to the center of rotation or axis of rotation about which the rotary member rotates in the testing operation. Furthermore, when the rotary member is clamped in position on a measuring spindle the testing machine between a flange on the spindle and the clamping mounting arrangement clampingly co-operating therewith, surface contact flaws or defects in respect of the flange or the clamping mounting arrangement can give rise to departures from the proper position of the rotary member on the spindle, which affects the condition of lateral run-out truth.
As the levels of requirement in respect of measuring accuracy when ascertaining radial and/or lateral run-out in relation to a rotary member are often in the region of a few hundredths of a millimetre, deviations and misalignments from a condition of perfect geometry of less than one hundredth of a millimetre must be maintained at the contact surfaces for supporting the rotary member on the spindle and at the location at which the rotary member is centered thereon. It is difficult in a practical context to achieve that kind of accuracy.
In regard to the levels of requirement in regard to measuring accuracy, it is also to be borne in mind that the rotational truth error or eccentricity of the clamping mounting arrangement for the rotary member exerts its effect fully circumferentially on radial run-out measurement in respect of the rotary member or test item, for example at beth wheel flanges of a motor vehicle disk wheel. That means that the measurement result suffers from errors in different ways both in regard to what is referred to as radial throw (peak-peak value) and also the first harmonics of radial run-out phenomena.
In addition the surface contact error in respect of the clamping mounting arrangement also causes falsification of the radial run-out measurement which has a different effect on the wheel flanges in the case of a disk wheel as they are arranged in relation to the mounting surface of the wheel in a position which is dependent on the depth of impression of the wheel, that is to say in dependence on the offset of the rim portion of the wheel relative to the wheel mounting center, and thus the wheel flanges are not generally symetrically relative to the wheel mounting surface. Because of the relationship between the mounting radius and the wheel flange radius which is always significantly greater than 1 and which is generally between about 3:1 and 5:1, the above-mentioned surface contact error can give rise to serious falsification in terms of lateral run-out measurement.
The mounting inaccuracies, in regard to radial or lateral run-out, always exert an overall influence which results from the combined action of the above-indicated phenomena, irrespective of whether the mounting error is pure eccentricity or a punctiform error or flaw. The influence on measurement in respect of the rotary member always takes its effect in the form of additive falsification in the radial or lateral run-out diagram produced. The configuration of radial and/or lateral run-out at the measuring locations, which is dependent on time and/or angle of rotation, is made up of error-affected first harmonics and non-error-affected upper harmonics and thus also determines the peak-peak value.